1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a separator interposed between single cells of a fuel cell.
2. Description of the Related Art
Various types of fuel cells exist, one of which is a polymer electrolyte fuel cell that is particularly well suited for use in vehicles due to its small size which is made possible by a low reaction temperature. This type of fuel cell is based on units of membrane electrode assemblies (MEAs), each MEA having a polymer electrolyte membrane sandwiched between two gas diffusion electrodes (each of which includes a catalyst layer and a porous support layer (i.e., a gas diffusion layer)). A separator that also acts as a supply channel for reaction gases such as hydrogen (i.e., the fuel gas) and oxygen (i.e., the oxidizing gas) is arranged on the outside of each membrane electrode assembly. The separator both acts as an impermeable barrier for the reaction gases, and collects power in order to extract the generated electric current to the outside. The MEA and separator together constitute one fuel cell unit. In an actual polymer electrolyte fuel cell, many of these fuel cell units are stacked together in series to form a cell module.
In order to maintain sufficient power generation efficiency with polymer electrolyte fuel cells, the electrolyte membrane must be kept sufficiently moist. Since the moisture from only the water produced by the electrolytic reaction is generally not sufficient, it is necessary to provide a mechanism to supply humidifying water to each MEA. Further, the electrolytic reaction generates heat of a heat quantity that substantially corresponds to the power generated, so a cooling mechanism must be used to prevent the fuel cell itself from heating up excessively.
For this cooling mechanism, the applicant proposes technology which mixes humidifying water in a mist state with the reaction gas to be supplied to the air electrode, such that it is supplied together with the reaction gas to the gas diffusion electrode. In attempt to improve manufacturability of the separator and make the fuel cell thinner, this technology employs a structure in which the separator is a corrugated (i.e., wavy) thin metal plate with air holes provided in a portion midway between the mountain peaks and the mountain bases in the corrugated plate. The reaction gas and the humidifying water which has been vaporized by heat from the separator are then supplied to the gas diffusion electrodes through these air holes. With this structure, the vaporization of the humidifying water within the gas supply passages can also be used for latent heat cooling.
Japanese Patent Laid-Open Publication No. 2002-184422 discloses one example of related art in which a separator of a polymer electrolyte fuel cell is made of a thin metal plate. This technology aims to maintain contact pressure following settling of the gas diffusion electrodes by employing a structure in which parallel slits are continuously formed in a metal plate serving as the separator. The portions between adjacent slits are then bent such that the metal plate is wavy shaped with offset phases, and thus is elastic. The cut portions created between the waves by the offset phases serve as air holes.
Also, Japanese Patent Laid-Open Publication No. 7-254424 proposes a fused carbonate fuel cell, which is a different type of fuel cell than the one described above, in which the collector plate (i.e., the separator) is formed of a corrugated thin plate, and a plurality of holes are provided through the thin plate at the top portion on the side that is in contact with the anode or the cathode.
With the technology disclosed in Japanese Patent Laid-Open Publication No. 2002-184422, lessening the pitch of the waves in order to ensure contact area reduces air flow, while increasing the pitch of the waves in order to ensure air flow conversely reduces the contact area. As a result, it is considered difficult to ensure both the contact area and the air flow at the same time. Further, in order to ensure the passage area, the waves must be at least a certain height, and in view of ensuring the contact pressure, the pitch of the waves has to be large.
Next, with the related technology disclosed in Japanese Patent Laid-Open Publication No. 7-254424, because the structure has holes for supplying gas provided only at the top portion of the thin collector plate that is in contact with the anode or cathode, the machining for opening the holes in the collector plate is done at the top portion of the waves, which is approximately only several millimeters wide, so it is considered a difficult operation that requires extreme skill.
Also, even if a structure in which holes are provided only at the top portion of the waves, as described above, can be applied with no problem to a fused carbonate fuel cell in which there is no diffusion layer on the electrodes, the same structure will pose problems if applied to a polymer electrolyte fuel cell in which a diffusion layer is provided on the electrodes. That is, in this structure, the holes must be positioned away from the curved portion of the wave and toward the inside, in the width direction, of the top portion to some extent. As a result, the aperture ratio may be restricted in some areas along the curved portion, in particular, of the contact surface that contacts the anode or cathode, such that the diffusion of the gas supplied may be uneven in view of the entire contact surface. Moreover, if the technology proposed earlier (i.e., mixing and supplying reaction gas and cooling water) by the applicant were to be applied to this technology, the cooling may be uneven, membrane moistening may be uneven, and furthermore, the supply of gas may be uneven due to clogging of the cooling water. Thus, problems exists with respect to the general applicability of the technology disclosed in Japanese Patent Laid-Open Publication No. 7-254424 to different types of fuel cells.
Furthermore, with a structure in which contact is made with a surface having a small aperture ratio (a surface with a large area), as described above, the contact surface must be extremely flat or else the actual power collecting surface will decrease due to only localized contact, which would result in higher power collection resistance.